Relation of concavity in the expiratory flow-volume loop to dynamic hyperinflation during exercise in COPD

https://doi.org/10.1016/j.resp.2016.08.005Get rights and content

Highlights

  • The spontaneous flow-volume curves become concave in COPD patients during exercise.

  • Some show marked, others moderate concavity depending on their resting FEV1.

  • The degree of concavity of the spontaneous flow-volume curves during exercise correlates with the developing dynamic hyperinflation in patients with COPD.

Abstract

Active expiration during exercise can increase intrathoracic pressure yielding concavity in the expiratory flow-volume loop in COPD. We investigated the relationship between this concavity and dynamic hyperinflation (DH). 17 COPD patients (FEV1: 38 ± 10%pred, GOLD stage 3–4) and 12 healthy subjects performed cycle ergometer incremental exercise. Expiratory limb of the spontaneous flow-volume loop was analyzed breath-by-breath using a geometric approach (rectangular area ratio (RAR), Respir. Med., 104(3):389–96, 2010). RAR below 0.5 demonstrates expiratory limb concavity. DH was determined with serial inspiratory capacity maneuvers. 5 of 17 patients displayed little end-exercise concavity (RAR = 0.52 ± 0.04, group LCONC). 12 patients had concavity at rest and end-exercise RAR reached 0.40 ± 0.03 (group HCONC). Healthy subjects showed no concavity. End-exercise RAR correlated with resting FEV1%pred (R2 = 0.81, P < 0.05). Group HCONC, compared to groups LCONC and H, reached significantly lower work rate, minute ventilation, and more dyspnea. DH inversely correlated with RAR (R2 = 0.81, P < 0.05). Detection of concavity in spontaneous flow-volume loops may help assess DH and exercise limitation in COPD.

Introduction

Exercise intolerance is one of the major symptoms contributing to declining quality of life in patients with COPD. The pathogenesis is multifactorial. One of the main contributors is reduced ventilatory capacity caused by expiratory airflow limitation (O'Donnell, 2006, O'Donnell et al., 2001, Porszasz et al., 2005).

COPD patients may experience airflow limitation both at rest and during exercise (Hyatt, 1961). Flow limitation occurs once airflow reaches a speed at which the flow is independent from pleural pressure, however, in emphysema the negative pressure dependence occurs at all lung volumes (Ingram and Schilder, 1966a, Ingram and Schilder, 1966b). Flow limitation can exacerbate breathing effort, leading to high intrathoracic pressure (Hyatt, 1961, Milic-Emili, 2000). Concomitantly, as ventilation increases during exercise and expiratory time decreases, expiratory airflow limitation makes it impossible to complete a full expiration to the relaxation volume. This yields increases in end-expiratory lung volume (EELV), a phenomenon known as dynamic hyperinflation (Casaburi and Porszasz, 2006, O'Donnell, 2006, O'Donnell et al., 2001, O'Donnell and Webb, 1993, Vogiatzis et al., 2004). As dynamic hyperinflation proceeds, end inspiratory volumes approach the total lung capacity where lower lung compliance yields higher inspiratory work and, eventually, inspiratory muscle fatigue and exercise-limiting dyspnea (Eltayara et al., 1996, O'Donnell and Webb, 1993).

O’Donnell et al. have provided confirmatory evidence correlating Borg dyspnea ratings and measurements of inspiratory capacity (IC) to dynamic hyperinflation and a decrease in endurance time during submaximal constant work rate cycle exercise. Their findings were highly reproducible and responsive to interventions reducing dynamic hyperinflation in severe COPD (O'Donnell et al., 1998).

We have previously reported a computerized technique enabling breath-by-breath analysis of the shape of the expiratory limb of the flow-volume loop (Ma et al., 2006, Ma et al., 2010). We intended to test the hypothesis that the observed rapid decrease in expiratory flow during spontaneous breathing during exercise correlates well with dynamic hyperinflation and determine whether this measure concavity of the expiratory limb of the spontaneous flow-volume loop signals imminent exercise termination in severe COPD patients. We also compared the differences in physiologic responses in groups with and without expiratory flow-volume loop concavity.

Section snippets

Study subjects

Seventeen patients (11 males) with severe to very severe COPD (FEV1: 38 ± 10%pred), and 12 healthy subjects (5 males) gave written informed consent for their participation in this study. The study was approved by the Los Angeles Biomedical Research Institute Institutional Review Board; all subjects were studied at this institution. Inclusion criteria for the COPD patients was a baseline post-bronchodilator FEV1 less than 60% predicted of normal by European Coal and Steel Standards (Quanjer et

Results

The demographic characteristics and resting spirometric function of the subjects are presented in Table 1. Among the COPD subjects, we noted two distinct patterns of configuration of the expiratory limb of the spontaneous flow-volume loop. At end-exercise, in 12 subjects severe concavity of the EFVL was seen (RAR = 0.40 ± 0.03, group HCONC), whereas in 5 subjects mild or no concavity (RAR = 0.52 ± 0.04, group LCONC) was present at end-exercise (Table 2); a dividing criterion of RAR = 0.47 at end-

Discussion

Concavity of the expiratory limb of the flow-volume loop is a marker of expiratory flow limitation. In this study, for the first time, we have shown that concavity of the expiratory limb of the spontaneous flow-volume loop is closely correlated with dynamic hyperinflation during exercise measured by serial inspiratory capacity maneuvers. Both dynamic hyperinflation and expiratory flow limitation are markers of ventilatory limitation. Determination of concavity of expiratory limb of the

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